Hoogle v5 uses a different json representation for the data, requires a
new URL and also contains HTML markup inside the json fields.

This commit removes all markup to be able to handle the hoogle responses
using the same structure as with the previous response structure (from
hoogle v4).

update jupyterlab installation procedure in README

Fix haddock generation.

Add ihaskell-graphviz to display graphs.

Requires 'Graphviz' installed.

Update .travis.yml

ihaskell-widgets: Fix more warnings

This file was giving a strange error/warning that had to be disabled.

After investigating this issue (https://ghc.haskell.org/trac/ghc/ticket/15601)
it turns out this was due to an wart in the way type level lists are handled
in GHC. Fortunately a workaround is available allowing the warning to be
re-enabled.

Fix more warnings!

GHC 8.4 and later have the `-Wpartial-fields` warning flag, so add it and
fix the warnings.

Turn on and fix more warnings

ihaskell-widgets: Make it compile with ghc 8.4

Fixing these warnings required changes to the file names of the `MessageHeader`
struct which makes this commit an API change. The version in the cabal file
has been bumped accordingly.

-Wall the things

Add `-Wunused-do-bind -Wunused-local-binds -Wunused-top-binds -Wunused-matches`
and fix all the things.

Add `-Wmissing-signatures` to `ghc-options`.

Add `-Wincomplete-patterns` to the `ghc-options` field of the cabal file
and fix all warnings.

GHC 8.4 allows value level operators (in this case list append, '++') to
be used at the type level, so that the singletons package for GHC 8.4
uses this operator whereas previous versions of singletons defined a ':++'
operator for type level append.